Methylmercury, a poisonous substance, is in the fish we eat. Thousand of tons of mercury are released into the air each year through pollution and waste. In the environment the mercury can transform into organic mercury, which is known as methylmercury, and accumulate in steams, oceans, water and soil.

Methylmercury also accumulates in the food chain, so each fish absorbs the mercury in other fish and organisms it eats. For this reason, larger and older fish such as shark and swordfish contain the highest levels of methylmercury.

People who regularly eat fish have higher levels of methylmercury than those who don't. Pregnant or breastfeeding women who eat a lot of fish put their newborns at risk, as methylmercury can harm a developing baby's brain and nervous system. Other particularly groups that are particularly sensitive to mercury exposure include children under the age of 6 years, people with impaired kidney function and people with sensitive immune responses to metals.

Both the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) have issued health advisories about consuming fish due to methylmercury contamination. The FDA recommends that pregnant women, nursing mothers and children limit their consumption of fish to 12 ounces per week and completely avoid shark, swordfish, king mackerel and tilefish (also known as golden bass and golden snapper). The EPA issued even stricter guidelines.

Particular concern has been raised about tuna since Americans consume so much of this fish. Canned tuna typically has lower levels of methylmercury than fresh tuna because canned tuna usually comes from smaller fish. However, since Americans eat canned tuna in high quantities, it could still pose a risk. It is therefore recommended that pregnant women, nursing mothers and children limit their consumption of tuna to no more than one 6-ounce can per week, as it is possible that more could result in neurological damages to babies and young children.

Methylmercury toxicity can result in paraesthesia (a tingling sensation on the skin), depression, and blurred vision. In fetuses and developing infants it can also have negative effects on attention span, language, visual-spatial skills, memory and coordination. It is estimated that nearly 60,000 children each year are born at risk for neurological problems due to methylmercury exposure in the womb.

For information about the mercury risk posed by specific fish, please see below.

Pregnant or nursing women and young children should avoid:

Shark, swordfish, tilefish (aka: golden bass, golden snapper), tuna steak (also used in sushi and sahimu)

Pregnant or nursing women and young children should limit:

Canned tuna, sea bass, gulf coast oysters, marlin, halibut, pike, walleye, white croaker, largemouth bass, Mahi Mahi, blue mussle, cod, eastern oyster, channel catfish (wild), Great Lakes salmon, Gulf Coast blue crab, lake whitefish, Pollack

The following fish are lowest in methylmercery:

Mid-Atlantic blue crab, croaker, fish sticks, flounder (summer), haddock, Wild Pacific salmon, shrimp